1. Field of the Invention
The present invention relates to a battery using a separator made of polyolefin and the like.
2. Description of the Related Art
In recent years, portable electronic device technology has been significantly developed. Accordingly, electronic devices such as mobile phones and notebook computers have started to be recognized as a basic technology supporting the highly sophisticated information society. Further, research and development for sophisticating these electronic devices has been actively promoted. In proportion to such sophistication, the power consumption of these electronic devices continues to be increased. On the other hand, these electronic devices are demanded to be driven for a long time. Therefore, a high energy density of the secondary battery as a driving power source has been consequently demanded.
In view of the occupied volume and the weight of the battery built in the electronic devices, a higher battery energy density is more desirable. In these days, since lithium ion secondary batteries have a superior energy density, the lithium ion secondary battery is built in most devices.
In general, in the lithium ion secondary battery, lithium cobaltate is used for the cathode, a carbon material is used for the anode, and the operating voltage is used in the range from 4.2 V to 2.5 V. In an electric cell, ability to increase the terminal voltage up to 4.2 V highly depends on superior chemical stability of the nonaqueous electrolyte material, the separator and the like.
In traditional lithium ion secondary batteries operating at 4.2 V at maximum, for the cathode active material such as lithium cobaltate used for the cathode, only about 60% of the capacity to the theoretical capacity is used. Therefore, in principle, it is possible to utilize the remaining capacity by further increasing the charging voltage. In fact, it is known that a high energy density is realized by setting the voltage in charging to 4.25 V or more (for example, refer to International Publication No. WO03/019713).